Harrat Khaybar
Harrat Khaybar is a volcanic field located in the north of Medina in Saudi ArabiaNASA

Australian archaeologist David Kennedy discovered around 400 ancient gates on old lava domes of Harrat Khaybar, north of Medina in Saudi Arabia in October, and now he has provided the clearest photos of the formations.

Kennedy was granted special permission to fly over the remote desert in Saudi Arabia's vast array of archaeological sites for the first time. The Saudi government lifted the veil and allowed him to conduct the aerial survey of the site after his research made headlines.

Helicopter flights gave him and his colleague Don Boyer — a geologist who now works in archaeology — access to the "extensive Harret Uwayrid (and contiguous Harret Raha) to the west, stretching some 77 miles (125 km) and rising to an elevation of about 6,300 feet (1,920 metres)," Kennedy, writing for LiveScience, said.

Apart from the gate structures, he also got the aerial views of kites, pendants, keyholes and other structures they had found on Google Earth. They flew for around 15 hours and took around 6,000 photos of 200 sites.

There is no obvious explanation why these structures, which differ in sizes, were built, but experts are of the opinion that this aerial photography could lead to a breakthrough one day.

"Because the area is so immense — encompassing some 10,000 square miles, or 27,000 square km — this is a task for remote sensing. This method will be combined with several techniques: The interpretation of Google Earth imagery systematically, the cataloging of the sites located, complementary low-level aerial reconnaissance and photography, and associated ground investigation," Kennedy said.

"Of course, Google Earth will remain a useful tool for prospection; it is simple to 'pin' and catalog sites, measure them, sketch them and generate distribution maps for interpretation. The limitations are equally obvious, however.

"The imagery is two-dimensional, and even the best resolution can be very fuzzy when enlarged. Detail is missing, and some sites are effectively invisible for various reasons. And imagery may be months, or even years, old and thus less valuable for routine monitoring of development."

The mysterious structures date back thousands of years and are located in the volcanic field of Harrat Khaybar. The 400 mysterious structures are called the "gates" because they looked like old-fashioned barred gates.

"The gates are stone-built, the walls roughly made and low," Kennedy wrote in the paper, which appeared in the November issue of Arabian Archaeology and Epigraphy. "The gates appear to be the oldest man-made structures in the landscape."

He added: "Identification, mapping and preliminary interpretation imply an early date in the sequence of the works — perhaps the very earliest — but no obvious explanation of their purpose can be discerned. 

"Gates are found almost exclusively in bleak, inhospitable lava fields with scant water or vegetation, places seemingly amongst the most unwelcoming to our species."